OXIDATION AND REDUCTION OF CHLOROPHYLL 463 



to point out that the absence of a visible brown phase does not necessarily 

 prove that no colorless intermediate product is formed in the alkali 

 cleavage of allomerized chlorophyll. This result may also be explained 

 by a slower rate of formation of the brown intermediate, combined with 

 an unchanged (or even accelerated) rate of its disappearance. 



It will be noted that, while the hydrogen atom in position 10 is 

 "labile" in the sense that it is easily shifted to the oxygen atom in 

 position 9, and allows an addition of oxygen or quinone, it is not neces- 

 sarily easily removable from the chlorophyll molecule. It was stated in 

 chapter 9 that the oxidation-reduction potential of an organic system of 

 the type RII2/R depends on the degree of resonance stabilization of the 

 double bond formed by oxidation. Even if the free energy of hydro- 

 genation is small or negative, the capacity for reversibly reduction may 

 be absent because of the high energy of the "odd" reduction intermediate. 

 Only those pairs RH2/R can act as reversible oxidation-reduction systems 

 whose intermediate radicals ("semiquinones") are stabilized by resonance 

 (page 231). Thus, the "lone" hydrogen atom which chlorophyll 

 possesses in position 10 could be available for easy, reversible oxidation- 

 reduction only if the radical formed by its removal (10-monodehydro- 

 chlorophyll) were stabilized by resonance. In equations (16.8) and 

 (16.10), the oxidation of the CH — group was supposed to be brought 

 about by additions and substitutions not involving the formation of free 

 radicals. Oxidants of moderate oxidation potential, acting by straight- 

 forward transfer of hydrogen atoms or electrons (c/. Chapter 9) are 

 likely to leave the CH — group unaffected, and to remove instead the 

 hydrogen atoms in positions 7 and 8, whose loss can be compensated for 

 by the formation of a double bond. They convert phorbides into the 

 corresponding porphyrins {i. e., remove two hydrogen atoms in nucleus 

 IV), leaving the cyclopentanone ring intact. Two hydrogen atoms can 

 be removed from chlorophyll, according to Conant, Dietz, Bailey, and 

 Kamerhng (1931), by potassium molybdicyanide, and according to Fischer 

 and Lautsch (1936) by silver oxide and silver acetate; with some chlorins, 

 the same effect can be achieved also by means of molecular oxygen if 

 copper acetate is used as a catalyst (Fischer and Herrle 1936). 



According to Fischer's interpretation of the nature of protochlorophyll, 

 the oxidation of chlorophyll by the above-mentioned dehydrogenizing 

 agents should lead to protochlorophyll (provided the hydrogen atoms in 

 positions 7 and 8 are removed without side reactions). A confirmation 

 of this conclusion would be of great interest. 



Even more interesting would be a direct reduction of protochlorophyll 

 back to chlorophyll. However, the conversion of porphyrins into chlorins 

 (or phorbins) by hydrogenation is one problem of chlorophyll synthesis 

 which has not yet been successfully solved. Thus, although the system 



